In recent years, there has been a demand for achieving both reduction in a nitrogen oxide (referred to as “NOx”) emission amount and improvement of a fuel consumption rate. PTL 1 discloses that when fuel injection timing (referred to as “SOI (Start Of Injection)”) is retarded, and a combustion temperature in a cylinder of an engine is lowered, an NOx emission amount is reduced, but when SOI is retarded, the fuel consumption rate is deteriorated, that is, the NOx emission amount and the fuel consumption rate are in a trade-off relationship with respect to change of the SOI.